feat(db): add imported_at column to media_items for accurate "Recently Added" sorting

The created_at column stores file modification time (intentional for preserving
original metadata), but this makes 'Recently Added' sorting unreliable for
imported files. Add imported_at column that records the actual database insert
timestamp.

Changes:
- Add imported_at TIMESTAMPTZ column to media_items (nullable)
- Update GetRecentlyAddedItems to sort by imported_at DESC NULLS LAST first
- Add ReassignLibraries and ReassignMediaItems queries for admin deletion handover
- Add SyncLibraryTypeExtensions query for startup extension sync
- Update all media_items SELECT queries to include imported_at column

internal/database/models.go

@@ -239,6 +239,7 @@ type MediaItems struct {
 	GoogleBooksID   pgtype.Text        `db:"google_books_id" json:"google_books_id"`
 	AddedByAdminID  pgtype.UUID        `db:"added_by_admin_id" json:"added_by_admin_id"`
 	CreatedAt       pgtype.Timestamptz `db:"created_at" json:"created_at"`
+	ImportedAt      pgtype.Timestamptz `db:"imported_at" json:"imported_at"`
 	UpdatedAt       pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
 	FormatGroup     string             `db:"format_group" json:"format_group"`
 	FormatMimetype  pgtype.Text        `db:"format_mimetype" json:"format_mimetype"`